Over the past 50 years, the field of organic photochemistry has produced a wealth of information, from reaction mechanisms to useful methodology for synthetic transformations. Many technological innovations have been realized during this time due to the exploits of this knowledge, including photoresists and lithography for the production of integrated circuits, photocharge generation for xerography, multidimensional fluorescence imaging, photodynamic therapy for cancer treatment, photoinitiated polymerization, and UV protection of plastics and humans through the development of UV absorbing compounds and sunscreens, to name a few.
The scientific basis of many of these processes continues to be utilized today, particularly in the development of organic three-dimensional optical data storage media and processes.
With the ever-pressing demand for higher storage densities, researchers are pursuing a number of strategies to develop three-dimensional capabilities for optical data storage in organic-based systems. Among the various strategies reported are holographic data storage using photopolymerizable media (Cheben, P. and Calvo, M. Appl. Phys. Lett. 2001, 78, 1490; U.S. Pat. No. 5,289,407 and U.S. Pat. No. 6,310,850), photorefractive polymers (Belfield et al. Field Responsive Polymers, ACS Symposium Series 726, ACS, 1999, Chapter 17), and two-photon induced photochromism (Belfield et al. Organic Photorefractives, Photoreceptors, and Nanocomposites, Proc. SPIE Vol. 4104, 2000, 15-22; U.S. Pat. No. 5,268,862). It is known that fluorescent properties of certain fluorophores may be changed (quenched) upon protonation by photogeneration of acid (Kim et al. Angew. Chem. Int. Ed. 2000, 39, 1780). Belfield et al. J. Phys. Org. Chem. 2000, 13, 837 has reported two-photon induced photoacid generation using onium salts and short pulsed near-IR lasers in the presence of a polymerizable medium, resulting in two-photon photoinitiated cationic polymerization. The inherent three-dimensional features associated with two-photon absorption provides an intriguing basis upon which to combine spatially-resolved, two-photon induced photoacid generation and fluorescence quenching with two-photon fluorescence imaging.
The quadratic, or nonlinear, dependence of two-photon absorption on the intensity of the incident light has substantial implications (dw/dt oc I2). For example, in a medium containing one-photon absorbing chromophores significant absorption occurs all along the path of a focused beam of suitable wavelength light. This can lead to out-of focus excitation. In a two-photon process, negligible absorption occurs except in the immediate vicinity of the focal volume of a light beam of appropriate energy. This allows spatial resolution about the beam axis as well as radially, which circumvents out-of-focus absorption and is the principle reason for two-photon fluorescence imaging (Denk et al. Science 1989, 248, 73). Particular molecules can undergo upconverted fluorescence through nonresonant two-photon absorption using near-IR radiation, resulting in an energy emission greater than that of the individual photons involved (upconversion). The use of a longer wavelength excitation source for fluorescence emission affords advantages not feasible using conventional UV or visible fluoresence techniques, e.g., deeper penetration of the excitation beam and reduction of photobleaching, and is particularly well-suited for fluorescence detection in multilayer coatings.
U.S. Pat. No. 5,268,862 reported two-photon induced photochromism of spiropyran derivatives at 1064 nm. Analogous to single-photon absorption facilitated isomerizion, the spiropyran underwent ring-opening isomerizion to the zwitterionic colored merocyanine isomer. The merocyanine isomer underwent two-photon absorption at 1064 nm, resulting in upconverted fluorescence. However, spiropyrans are known to undergo photobleaching and photodegradation upon prolonged exposure with blurring effects observed outside the irradiated volume, and hence are not suitable for long-term use. U.S. Pat. No. 5,253,198 disclosed a bacteriorhodopsin-based holographic recording media and process, using two-photon excitation. High data storage and photostabilities were repored for this rather complex system, however it requires near-zero gravity conditions for processing to ensure homogeneous distribution of the bacteriorhodopsin, an electrochemical system to measure the electrical response vs. a purely optical response, and careful handling of the biological material (the protein). Though the read time claimed of 100 ns is impressive, as are read data rates of up to 10 Mbit/sec, the complexity of the system seriously undermines any practical potential applications of the system.
Thus, in addition to high data storage volume and fast readout, there is a need for data storage materials that are stable, highly responsive, exhibit high sensitivity and fidelity, and are less complex. In addition, the data storage and readout processes must also be more straight forward (less complex) and reliable. As mentioned above, the previously developed systems fall short in these regards.